Sample and hold circuits play an important role in data acquisition systems, particularly in those systems in which the signals containing the data of interest are changing faster than the system can acquire and appropriately process the data.
For example, in large area, flat panel imaging systems, such as imaging systems for medical and document imaging applications based upon amorphous silicon, the image sensor is typically arranged as an array of pixels, each of which consists of a photosensitive element and a thin film transistor (TFT). In order to achieve imaging frame rates suitable for video processing and display, all gate and data line connections for the sensor are brought out to the edge of the array for connection to an off-array control circuit containing row selection and charge sensing circuitry. For a high resolution array, many pixels are used for each data line, with the result being a high data readout rate in order to sample each of the pixels within the time constraints of the real time video display. Accordingly, while the data for each pixel must be sampled accurately, it must also be sampled quickly and held available for a sufficient period of time to allow the pixel data to be appropriately processed, stored, etc.
However, such image data signals, due to the manner in which they are generated, include, in addition to the image component, a noise component which is generated as a result of the image array scanning process. For example, the circuitry used to acquire the image data information from each pixel typically includes a charge sensitive pre-amplifier which must be reset between each pixel. This resetting of the charge sensitive pre-amplifier immediately prior to reading out the charge from each pixel generates a significant, and undesirable, noise component which, if not eliminated during the sample and hold process, will significantly distort and obscure the true image information corresponding to that pixel.
Accordingly, it would be desirable to have a sample and hold circuit which is capable of eliminating the noise component from the signal to be sampled.